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<p>Defines an <a href="expressions.html" title="cpp/language/expressions">expression</a> that can be evaluated at compile time.
</p>
<p>Such expressions can be used as non-type template arguments, array sizes, and in other contexts that require constant expressions, e.g.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<a href="../container/array.html"><span class="kw1062">std::<span class="me2">array</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, n<span class="sy1">&gt;</span> a1<span class="sy4">;</span> <span class="co1">// error, n is not a constant expression</span>
<span class="kw4">const</span> <span class="kw4">int</span> cn <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<a href="../container/array.html"><span class="kw1062">std::<span class="me2">array</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span>, cn<span class="sy1">&gt;</span> a2<span class="sy4">;</span> <span class="co1">// OK, cn is a constant expression</span></pre></div></div>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="constant_expression.html#Core_constant_expressions"><span class="tocnumber">1</span> <span class="toctext">Core constant expressions</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="constant_expression.html#Constant_expression"><span class="tocnumber">2</span> <span class="toctext">Constant expression</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="constant_expression.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="constant_expression.html#See_also"><span class="tocnumber">4</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Core constant expressions">edit</a>]</span> <span class="mw-headline" id="Core_constant_expressions">Core constant expressions</span>
</h3>
<p>A <i>core constant expression</i> is any expression that <i>does not</i> have any one of the following in any subexpression (ignoring the unevaluated arguments of <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&amp;&amp;</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span>, and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">?:</span></span></span>)
</p>
<div class="t-li1">
<span class="t-li">1)</span> A function call expression that calls a function (or a constructor) that is not declared <a href="constexpr.html" title="cpp/language/constexpr">constexpr</a> 
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">constexpr</span> <span class="kw4">int</span> n <span class="sy1">=</span> <a href="../types/numeric_limits.1.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, max() is constexpr</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> m <span class="sy1">=</span> <a href="../chrono/c/time.html"><span class="kw855">std::<span class="me2">time</span></span></a><span class="br0">(</span><a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: std::time() is not constexpr</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">2)</span> A function call to a <code>constexpr</code> function which is declared, but not defined, if the function call itself is not made inside the body of a <code>constexpr</code> function.</div>
<div class="t-li1">
<span class="t-li">3)</span> A function call to a <code>constexpr</code> function with arguments that do not produce a constant expression when substituted
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">constexpr</span> <span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> addr<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> ir<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy3">&amp;</span>ir<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span>
<span class="kw4">constexpr</span> <span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> xp <span class="sy1">=</span> addr<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="kw4">constexpr</span> <span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span> tp <span class="sy1">=</span> addr<span class="br0">(</span><span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: &amp;5 is not a constant expression</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">4)</span> A function call to a <code>constexpr</code> function with arguments that do not produce constant expressions in member-initializer lists that are called from this function
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x<span class="sy4">;</span>
<span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw4">constexpr</span> A<span class="br0">(</span><span class="kw4">bool</span> b<span class="br0">)</span> <span class="sy4">:</span> m<span class="br0">(</span>b<span class="sy4">?</span><span class="nu0">42</span><span class="sy4">:</span>x<span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
    <span class="kw4">int</span> m<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> v <span class="sy1">=</span> A<span class="br0">(</span><span class="kw2">true</span><span class="br0">)</span>.<span class="me1">m</span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> w <span class="sy1">=</span> A<span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span>.<span class="me1">m</span><span class="sy4">;</span> <span class="co1">// error: non-const x</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">5)</span> A function call to a recursive <code>constexpr</code> function that would exceed compile-time recursion depth limit (implementation-defined)</div>
<div class="t-li1">
<span class="t-li">6)</span> The <a href="this.html" title="cpp/language/this">this</a> pointer except if used for class member access inside a non-static member function.</div>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx14">
<td>
<div class="t-li1">
<span class="t-li">7)</span> An expression whose result is not mathematically defined or is out of range for its type.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">constexpr</span> <span class="kw4">double</span> d1 <span class="sy1">=</span> <span class="nu16">2.0</span><span class="sy2">/</span><span class="nu16">1.0</span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="kw4">constexpr</span> <span class="kw4">double</span> d2 <span class="sy1">=</span> <span class="nu16">2.0</span><span class="sy2">/</span><span class="nu16">0.0</span><span class="sy4">;</span> <span class="co1">// Error: not defined</span>
<span class="kw4">constexpr</span> <span class="kw4">int</span> n <span class="sy1">=</span> <a href="../types/numeric_limits.1.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// Error: overflow</span></pre></div></div>
</div>
</td>
<td><span class="t-mark-rev t-until-cxx14">(until C++14)</span></td>
</tr>
<tr class="t-rev t-since-cxx14">
<td>
<div class="t-li1">
<span class="t-li">7)</span> An expression whose evaluation leads to any form of <span class="t-rev-inl t-since-cxx17"><span>core language</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> undefined behavior (including signed integer overflow, division by zero, pointer arithmetic outside array bounds, etc). <span class="t-rev-inl t-since-cxx17"><span>Whether standard library undefined behavior is detected is unspecified.</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>
</div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
</tr>
</table>
<div class="t-li1">
<span class="t-li">8)</span> A <a href="lambda.html" title="cpp/language/lambda">lambda expression</a>
</div>
<div class="t-li1">
<span class="t-li">9)</span> An lvalue-to-rvalue <a href="implicit_cast.html" title="cpp/language/implicit cast">implicit conversion</a> applied to a non-active member of a <a href="union.html" title="cpp/language/union">union</a> or its subobject</div>
<div class="t-li1">
<span class="t-li">10)</span> Any other lvalue-to-rvalue <a href="implicit_cast.html" title="cpp/language/implicit cast">implicit conversion</a>, unless the lvalue...</div>
<div class="t-li2">
<span class="t-li">a)</span> has integral or enumeration type and refers to a <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">complete</a> non-volatile const object, which is initialized with a constant expression</div>
<div class="t-li2">
<span class="t-li">b)</span> is a non-volatile glvalue that refers to an element of a string literal</div>
<div class="t-li2">
<span class="t-li">c)</span> has literal type and refers to a non-volatile object defined with <code>constexpr</code> or to its non-mutable subobject</div>
<div class="t-li2">
<span class="t-li">d)</span> has literal type and refers to a non-volatile temporary, initialized with a constant expression</div>
<div class="t-li1">
<span class="t-li">11)</span> a reference to a variable unless it was initialized with a constant expression or is a non-static data member of a temporary object whose lifetime begain within this expression</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14">
<td>
<div class="t-li1">
<span class="t-li">12)</span> conversion from <i>cv</i> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span></span></span> to any pointer-to-object type
</div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
</tr>
</table>
<div class="t-li1">
<span class="t-li">13)</span> <a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a>
</div>
<div class="t-li1">
<span class="t-li">14)</span> <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a>
</div>
<div class="t-li1">
<span class="t-li">15)</span> pseudo-destructor call</div>
<div class="t-li1">
<span class="t-li">16)</span> an increment or a decrement operator</div>
<div class="t-li1">
<span class="t-li">17)</span> a <a href="typeid.html" title="cpp/language/typeid">typeid</a> expression applied to a glvalue of polymorphic type</div>
<div class="t-li1">
<span class="t-li">18)</span> a new-expression or a delete-expression</div>
<div class="t-li1">
<span class="t-li">19)</span> a subtraction operator between two pointers</div>
<div class="t-li1">
<span class="t-li">20)</span> an equality or relational operator when the result is unspecified</div>
<div class="t-li1">
<span class="t-li">21)</span> an assignment or a compount assignment operator</div>
<div class="t-li1">
<span class="t-li">22)</span> a throw expression</div>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: needs more mini-examples and less standardese </td>
</tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Constant expression">edit</a>]</span> <span class="mw-headline" id="Constant_expression">Constant expression</span>
</h3>
<ul><li> <i>Integral constant expression</i> is an expression of integral or unscoped enumeration type implicitly converted to a prvalue, where the converted expression is a core constant expression. If an expression of class type is used where an integral constant expression is expected, the expression is <a href="implicit_cast.html" title="cpp/language/implicit cast">contextually implicitly converted</a> to an integral or unscoped enumeration type. 
</li></ul>
<dl><dd> only integral constant expressions can be used as <span class="t-rev-inl t-until-cxx14"><span><a href="array.html" title="cpp/language/array">array bounds</a>, the dimensions in <a href="new.html" title="cpp/language/new">new-expressions</a> other than the first</span> <span><span class="t-mark-rev t-until-cxx14">(until C++14)</span></span></span>, <a href="bit_field.html" title="cpp/language/bit field">bit-field lengths</a>, enumeration initializers when the underlying type is not fixed, <span class="t-rev-inl t-until-cxx14"><span>null-pointer constants</span> <span><span class="t-mark-rev t-until-cxx14">(until C++14)</span></span></span>, and alignments.
</dd></dl>
<ul><li> <i>Converted constant expression</i> is an expression <a href="implicit_cast.html" title="cpp/language/implicit cast">implicitly converted</a> to prvalue of type T, where the converted expression is a core constant expression, and the implicit conversion sequence contains only:
</li></ul>
<dl><dd>
<ul>
<li> constexpr user-defined conversions (so a class can be used where integral type is expected)
</li>
<li> lvalue-to-rvalue conversions
</li>
<li> integral promotions
</li>
<li> non-narrowing integral conversions
</li>
</ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17">
<td>
<dl><dd>
<ul>
<li> array-to-pointer conversions
</li>
<li> function-to-pointer conversions
</li>
<li> qualification conversions
</li>
<li> null pointer conversions from <span class="t-lc"><a href="../types/nullptr_t.html" title="cpp/types/nullptr t">std::nullptr_t</a></span>
</li>
<li> null member pointer conversions from <span class="t-lc"><a href="../types/nullptr_t.html" title="cpp/types/nullptr t">std::nullptr_t</a></span>
</li>
</ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td>
</tr>
</table>
<dl>
<dd>
<ul><li> And if any <a href="reference_initialization.html" title="cpp/language/reference initialization">reference binding</a> takes place, it is direct binding (not one that constructs a temporary object)
</li></ul>
</dd>
<dd> Only <i>converted constant expressions</i> can be used as <a href="switch.html" title="cpp/language/switch">case expressions</a>, <a href="enum.html" title="cpp/language/enum">enumerator initializers</a> when the underlying type is fixed, <span class="t-rev-inl t-since-cxx14"><span><a href="array.html" title="cpp/language/array">array bounds</a>, the dimensions in <a href="new.html" title="cpp/language/new">new-expressions</a> other than the first</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>, and as <span class="t-rev-inl t-until-cxx17"><span>integral and enumeration </span> <span><span class="t-mark-rev t-until-cxx17">(until C++17)</span></span></span>non-type <a href="template_parameters.html" title="cpp/language/template parameters">template arguments</a>.
</dd>
</dl>
<ul>
<li> <i>Literal constant expression</i> is a prvalue <i>core constant expression</i> of non-pointer literal type (after conversions as required by context). A literal constant expression of array or class type requires that each subobject is initialized with a constant expression.
</li>
<li> <i>Reference constant expression</i> is an lvalue core constant expression that designates an object with static storage duration or a function.
</li>
<li> <i>Address constant expression</i> is a prvalue core constant expression (after conversions required by context) of type <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</span></span></a></span></span> or of a pointer type, which points to an object with static storage duration, one past the end of an array with static storage duration, to a function, or is a null pointer.
</li>
</ul>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span>
</h3>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14">
<td>Implementations are not permitted to declare library functions as <code>constexpr</code> unless the standard says the function is <code>constexpr</code> </td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td>
</tr> 
</table>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr>
<td class="mbox-empty-cell"></td>
<td class="mbox-text" style="">This section is incomplete<br>Reason: examples </td>
</tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<ul>
<li> <a href="constexpr.html" title="cpp/language/constexpr">constexpr specifier</a>
</li>
<li> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/is_literal_type.html"><span class="kw467">std::<span class="me2">is_literal_type</span></span></a></span></span>
</li>
</ul>

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